I think there are two possible 4 spds you could have SM465 or NP435, either has larger input shaft bearing retainer than the SM420 you are replacing. You can have your bell housing machined to fit or get the 68-72 bell. Here's a link to identify the trans.

http://www.novak-adapt.com/knowledge/transmission_gateway.htm

The only rear axle swap I have done is a 73 into my 68 and both were leaf. The biggest problem I had was with the shock mounts. Chevy changed the mounts to front and back, from both back. You may not have that problem since your lower shock mounts are brackets held on by the u-bolts on the control arms. I assume you have measured the axles outside and inside and there is no fit problem with any rims you have or might get. You will have to move the control arm and panhard mounts from the old axle to the new axle. Hopefully someone can add more.

Larry

I don't own a vehicle that isn't old enough to drink.